Re: Merging submodules - best merge-base

[Heiko Voigt <hvoigt@hvoigt.net>]

On Sat, Mar 09, 2013 at 06:45:56PM +0100, Jens Lehmann wrote:
> Am 07.03.2013 19:59, schrieb Heiko Voigt:
> > On Thu, Mar 07, 2013 at 10:49:09AM +0100, Daniel Bratell wrote:
> >> Den 2013-03-06 19:12:05 skrev Heiko Voigt <hvoigt@hvoigt.net>:
> >>> So to summarize what you are requesting: You want a submodule merge be
> >>> two way in the view of the superproject and calculate the merge base
> >>> in the submodule from the two commits that are going to be merged?
> >>>
> >>> It currently sounds logical but I have to think about it further and
> >>> whether that might break other use cases.
> >>
> >> Maybe both could be legal even. The current code can't be all wrong,
> >> and this case also seems to be straightforward.
> > 
> > Ok I have thought about it further and I did not come up with a simple
> > (and stable) enough strategy that would allow your use case to merge
> > cleanly without user interaction.
> > 
> > The problem is that your are actually doing a rewind from base to both
> > tips. The fact that a rewind is there makes git suspicious and we simply
> > give up. IMO, thats the right thing to do in such a situation.
> > 
> > What should a merge strategy do? It infers from two changes what the
> > final intention might be. For submodules we can do that when the changes
> > on both sides point forward. Since thats the typical progress of
> > development. If not there is some reason for it we do not know about. So
> > the merge gives up.
> > 
> > Please see this post about why we need to forbid rewinds from the
> > initial design discussion:
> > 
> > http://article.gmane.org/gmane.comp.version-control.git/149003
> 
> I agree that rewinds are a very good reason not merge two branches using
> a fast-forward strategy, but I believe Daniel's use case is a (and maybe
> the only) valid exception to that rule: both branches contain *exactly*
> the same rewind. In that case I don't see any problem to just do a fast
> forward to S21, as both agree on the commits to rewind.

That is different than using the merge base of the two commits needing
merge. I agree that rewinding to exactly the same commits is probably a
valid exception. Will have a look into extending the submodule merge
strategy to include this case.

Cheers Heiko